Beam steering LADAR sensor

ABSTRACT

In one embodiment, a ladar system includes a laser transmitter with at least one semiconductor laser having a pulsed laser light output. A laser drive circuit is connected to said at least one semiconductor laser and adapted to electrically drive said at least one semiconductor laser in a predetermined sequence. A laser beam steering mechanism is adapted to scan the pulsed laser light output sequentially through the field of view.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of patent application Ser. No.14/656,936, filed Mar. 13, 2015, which is hereby incorporated byreference.

TECHNICAL FIELD

The embodiments disclosed herein relate generally to 3-D imagegeneration and the identification and tracking of objects, and moreparticularly to LADAR sensors for mobile applications such as roadhazard avoidance, collision avoidance, and autonomous navigation. Theinvention compensates for the issues arising from the operation of aladar sensor having a limited optical power output.

BACKGROUND

The 3-D imaging technology disclosed in Stettner et al, U.S. Pat. Nos.5,446,529, 6,133,989 and 6,414,746 provides with a single pulse oflight, typically pulsed laser light, all the information of aconventional 2-D picture along with the third dimensional coordinates;it furnishes the 3-D coordinates of everything in its field of view.This use is typically referred to as flash 3-D imaging in analogy withordinary digital 2-D cameras using flash attachments for a selfcontained source of light. As with ordinary 2-D digital cameras, thelight is focused by a lens on the focal plane of the LADAR sensor, whichcontains an array of pixels called a focal plane array (FPA). In thecase of a LADAR sensor these pixels are “smart” and can collect datawhich enables a processor to calculate the round-trip time of flight ofthe laser pulse to reflective features on the object of interest.

Many systems have been proposed to meet the challenge of using opticalimaging and video cameras in a vehicle system to create 3-D maps ofscenes and models of solid objects, and to use the 3-D database tonavigate, steer, and avoid collisions with stationary or moving objects.Stereo systems, holographic capture systems, and those which acquireshape from motion, have not been able to demonstrate adequateperformance in this application, but 3D LADAR based systems have shownthe ability to rapidly capture 3-D images of objects and roadwayfeatures in the path of a moving vehicle, or travelling on anintersecting path, with sufficient speed and accuracy to allow the hostvehicle to avoid collisions and road hazards, and steer the best path.In order to produce a low cost and rugged design, it is foreseeablethere will be a need to use a semiconductor type laser pulse transmitterhaving a limited optical output power.

It is therefore desirable to provide a LADAR sensor capable of operatingwith a low power semiconductor laser array to illuminate the field ofview of the LADAR sensor. It is further desirable that the LADAR sensoris capable of mapping the entire area surrounding the vehicle, andallows for the avoidance of other moving vehicles, road hazards, andpedestrians.

BRIEF SUMMARY

A ladar sensor according to the present embodiments incorporates apulsed semiconductor laser to illuminate a scene in the field of view ofthe ladar sensor. Typically, a single very powerful laser pulse isoptically diffused across the entire field of view by a fixed diffusingoptic. The reduced power of the semiconductor laser may be offset byimproved sensitivity of the ladar receiver, or by spatiallyconcentrating the output of the semiconductor laser, and sweeping theconcentrated beam across the field of view. A focal plane array ofoptical detectors is positioned behind a light receiving and focussinglens, and a readout integrated circuit is connected to the electricaloutputs of the detectors of the focal plane array. Within the readoutintegrated circuit, an array of unit cell electrical circuits, amplifiesand detects incoming light pulses which have been converted toelectrical pulses in the detector elements of the focal plane array.Each unit cell electrical circuit is connected to a high speed timingclock, and within each unit cell is a digital timer. The digital timeris started counting by the flash of the scene illuminating opticalpulse, and counts the number of cycles of the timing clock. The digitalcounter is frozen at the time of detection of an electrical pulse in theunit cell, which happens at the time of arrival of an incoming opticalpulse. Thus the range to each reflective surface within the field ofview of the ladar system may be sensed and digitally measured.

In alternative embodiments, each ladar sensor may include an opticalgain element in the optical receiver path, between the lightconcentrating and focussing lens, and the focal plane array of opticaldetectors. This optical gain element may be optically pumped, as in theexample of an erbium doped fiber amplifier, or electrically pumped as inthe case of a semiconductor optical amplifier.

In other embodiments, each ladar sensor may have a focal plane array ofdetectors of indium gallium arsenide formed on a substrate of indiumphosphide, gallium arsenide, or silicon. In a further embodiment, anelectrical amplifier array may be interposed between the focal planearray of optical detectors and the readout integrated circuit. In yetanother embodiment, the electrical amplifier array and the readoutintegrated circuit may be provided with through hole vias through thesubstrate to eliminate wirebonds, and thereby enable the low cost andlow parasitic packaging of the focal plane array and readout integratedcircuit.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments of the present disclosureor may be combined in yet other embodiments, further details of whichcan be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a typical optical overload scenario with a firstladar sensor at the front right of a vehicle receiving strong light backscattered from a retroreflector embedded in the pavement, and from astop sign, which also has retroreflecting elements embedded in thesurface;

FIG. 2 shows one solution to the optical overload scenario illustratedin FIG. 1. Two forward looking ladar sensors are positioned at the frontof the vehicle, and the ladar sensor on the right, closest to the angleof the retroreflectors, responds to a laser pulse transmitted by theleftmost ladar sensor. Bidirectional connections are shown between theladar sensors which give the rightmost ladar sensor a precise time zeroreference signal indicating the transmission of a scene illuminatinglaser pulse from the leftmost ladar sensor, and vice versa;

FIG. 3 is a block diagram showing the elements of a typical vehicleinstallation, including the vehicle electrical systems, CPU, and thesubsystems which regulate the vehicle suspension, provide an inertialnavigation reference, provide global positioning references, makedecisions to deploy airbags, and communicate via duplex radio link tothe outside world;

FIG. 4 is a block diagram of a beam steering LADAR sensor whichdescribes the functions and connections between the ladar sensor controlprocessor, and subsystems for creating laser illuminating pulses,receiving the reflected laser pulses, reducing the data, storing theimages, and identifying objects within the image data sets;

FIG. 5 is a block diagram of a unit cell of the readout integratedcircuit (ROIC) of FIG. 4;

FIG. 6A is a cutaway side view of a beam steering mechanism featuring anedge emitting laser, a fixed mirror angle block, and a MEMS style micromirror capable of deflection in two axes;

FIG. 6B is a cutaway side view of a beam steering mechanism featuringtwo edge emitting lasers, two rod lenses, and two MEMS style micromirrors capable of deflection in two axes;

FIG. 6C is a cutaway side view of a beam steering mechanism featuring anedge emitting laser, a rod lens, and a cascade of two MEMS style micromirrors capable of deflection in two axes;

FIG. 6D is a cutaway side view of a beam steering mechanism featuring avertical cavity surface emitting laser, a silicon substrate, and a MEMSstyle micro mirror capable of deflection in two axes;

FIG. 6E shows an isometric view of the structure shown in FIG. 6B, andthe pixellated far field pattern which will be swept by the pulsed lasertransmitter.

FIG. 7A is an isometric view of an optical gain block showing a fusedfiber bundle of erbium doped fibers with a diffusing optical flat bondedto each of four sides;

FIG. 7B is an isometric view of an optical gain block showing theassembly of an array of vertical cavity surface emitting lasers (VCSEL)to one of four sides;

FIG. 7C is a top view of the optical gain block of FIG. 7C showing afused fiber bundle of erbium doped fibers with a diffusing optical flatbonded to each of four sides, and a VCSEL array bonded to each of thefour sides;

FIG. 8 is a cutaway side view of a ladar sensor having an optical gainblock of fused fibers which are optically pumped by a VCSEL arraycoupled through a diffusing element and a 45° dichroic mirror;

FIG. 9 is a cutaway side view of a ladar sensor having an array ofsemiconductor optical amplifiers (SOAs) positioned in the lightreceiving path;

FIG. 10A is a cutaway side view of an individual element of the array ofsemiconductor optical amplifiers (SOAs) of FIG. 9;

FIG. 10B is a cutaway side view of an individual element of the array ofsemiconductor optical amplifiers (SOAs) of FIG. 9, showing the formationon an optical flat;

FIG. 11 is a cutaway side view of a hybrid focal plane array (FPA)employing an InGaAs detector array which has been grown on a metamorphicepitaxial layer, which is grown on an inexpensive GaAs substrate. Thedetector array is mounted to a readout integrated circuit (ROIC), whichhas a number of front to back interconnects made by through thesubstrate vias.

FIG. 12 is a cutaway side view of a hermetic package containing thehybrid focal plane array (FPA) of FIG. 11;

FIG. 13 is a cutaway side view of a hybrid focal plane array (FPA)employing an InGaAs detector array which has been grown on a metamorphicepitaxial layer, grown on an inexpensive GaAs substrate. The detectorarray is mounted to an electrical amplifier array having front to backelectrical contacts made by through the substrate vias (TSVs). Theamplifier array is mounted to a substrate which has a hermetic windowcover. On the reverse side of the substrate is mounted a readoutintegrated circuit (ROIC).

FIG. 14 is a cutaway side view of a hybrid focal plane array (FPA)employing an array of discrete InGaAs detectors mounted to an electricalamplifier array having front to back electrical contacts made by throughthe substrate vias. The amplifier array is mounted to a printed circuitsubstrate. On the reverse side of the circuit substrate is mounted areadout integrated circuit (ROIC).

FIG. 15A shows a silicon wafer being bonded to a wafer of indiumphosphide having an epitaxial layer of indium gallium arsenide suitablefor P-intrinsic-N (PIN) detector formation;

FIG. 15B shows the indium phosphide wafer bonded to the silicon waferwith a major portion of the indium phosphide wafer thickness having beenremoved;

FIG. 15C shows the formation of isolated mesas of a PIN structure in theindium gallium arsenide epitaxial layer by creation of trenches in theindium phosphide and epitaxial layer of indium gallium arsenide;

FIG. 15D shows the formation of both anode and cathode contacts on theisolated PIN mesa structures;

FIG. 16 shows a top view of the detector array of FIG. 16D mounted to asupporting circuit substrate, and having a number of decouplingcapacitors mounted along the length;

FIG. 17 shows a schematic for a pixel amplifier element of the amplifierarray of FIGS. 13-14 and including a detector element of a detectorarray of the types described in FIGS. 4, 5, 8, and 11-16;

DETAILED DESCRIPTION

This application contains new subject matter related to previous U.S.Pat. Nos. 5,696,577, 6,133,989, 5,629,524, 6,414,746, 6,362,482,D463,383, and U.S. patent application Ser. No. 10/066,340 filed on Jan.31, 2002 and published as US 2002/0117340 A1, the disclosures of whichare incorporated herein by reference.

The embodiments disclosed herein enable a compact imaging ladar systemhaving improved performance. The applications for such a system may bein automotive collision avoidance and autonomous navigation, terrainmapping, landing and docking, and 3D movie/graphics capture. Theimprovements include an optical gain block which may be constructed froman array of erbium doped fibers, or formed in a semiconductor. In amulti-ladar installation, a facility to coordinate the illuminatingpulses of a first ladar transmitter with the receiver of a second ladaris described which may reduce the effects of saturation due to a cornerreflector in the field of view. Additionally, a number of improvementsto the detector array subassembly are shown. The hybrid assembly ofdetector array and readout integrated circuit (ROIC) incorporates anumber of novel features. The use of a separate analog amplifier arraysituated between the detector array and readout IC allows for enhancedsystem responsivity. Through silicon vias (TSVs) are used on theamplifier array and on the readout IC to enable a compact hybrid focalplane array (FPA) assembly which is inexpensive to assemble and hashigher electrical performance due to a reduction in parasitic inductancein the package. Two new detector technologies are also described. A PINdetector structure of indium gallium arsenide is formed on a strainedmetamorphic layer over a gallium arsenide substrate, allowing for theprocessing of inexpensive GaAs wafers up to 6″ in diameter. The seconddetector structure involves an InGaAs PIN structure bonded at the waferlevel to an inexpensive silicon substrate. This technology allows forextended thermal performance, and standard processes for solder bumpinterconnect. The use of a laser beam steering mechanism allows theladar sensor to illuminate the field of view in a number of sequentialsteps, reducing the peak power requirement for the laser beam. The useof a beam steering mechanism enables the use of pulsed semiconductorlasers, which are known to have lower power output than their solidstate counterparts. A number of laser beam steering designs aredescribed which enable a ladar sensor to use a lower power pulsedsemiconductor laser, and scan the beam sequentially through the field ofview.

A vehicle mounted ladar system may comprise a number of side mounted,rear mounted, or forward looking ladar sensors of the type describedherein. These ladar sensors can be connected to a central ladar systemcontroller which synthesizes the available data from each of theindependent ladar sensors into a composite 3D map of the immediate areain a full 360 degree arc surrounding the vehicle. In a preferredembodiment, conventional 2D still images or video sequences may be usedto improve the quality of 3D solid models and scene maps.

In a vehicle application, the ladar sensor may be incorporated into aheadlight, taillight, or other auxiliary lamp assembly. The ladar sensormay also be part of a backup light, rearview mirror assembly, or mountedbehind an opening in a bumper or grill assembly, or may be high mountedbehind the windshield, on a roof support, or in a modular assemblymounted through a cutout in a body panel at the periphery of thevehicle. The ladar sensor typically incorporates a hybrid assembly offocal plane array and readout integrated circuit, and the readout IC isarranged as an array of unit cell electrical circuits, and each unitcell is configured to fit in an array of identical spacing and order asthe mating focal plane array (FPA). The ladar sensor in a preferredembodiment is capable of working in a flash mode as described above, orin a multi-pulse mode, or in a pulsed continuous-wave mode as thesituation dictates. The ladar system incorporating the ladar sensor mayalso have a number of features which enable full 3D object modeling andtracking, as well as scene enhancements derived from the merging of 2Dand 3D data bases and managing of both 3D ladar sensors and conventional2D video cameras.

Each of the light sensitive detectors of the FPA has an output producingan electrical response signal from a reflected portion of the laserlight output. The electrical response signals are connected to a readoutintegrated circuit (ROIC) with a corresponding array of unit cellelectrical circuits. Each of the unit cell electrical circuits has aninput connected to one of the light sensitive detector outputs, anelectrical response signal amplifier and a demodulator, and a rangemeasuring circuit connected to an output of the electrical responsesignal demodulator. The demodulator may be a voltage sampler and analogshift register for storing sequential samples of the electrical responsesignals, or it may comprise a mixer, integrator, or matched filter. Inthe sampling mode, each unit cell uses a reference clock to time thesamples being taken in response to the captured reflection of the laserlight from a target surface. The demodulation may also take placeexternal to the readout integrated circuit, by a fast digital processoroperating on a sequence of digitized samples from each pixel. The fastdigital processor may employ algorithms which utilize weighted sums ofsequential analog samples, or use fast Fourier transforms, convolution,integration, differentiation, curve fitting, or other digital processeson the digitized analog samples of the electrical response. The fastdigital processor may also employ algorithms which isolate or segmentthe roadway from other objects and objects from each other. Such objectsmay be automobiles, bicycles, motorcycles, trucks, persons, animals,walls, signs, road obstructions etc. These algorithms may computeposition and orientation, as well as object velocity. Objects, theirorientation, position and velocity may be transferred to a centralcomputer for further processing and decision making. Each unit cellcircuit has the ability to preserve the shape of the returned ladarpulse, and to make inferences about the shape of the surface within apixel boundary as seen projected at a distance from the focal planearray, based on the shape of the reflected light pulse. The rangemeasuring circuit is further connected to a reference signal providing azero range reference for the modulated laser light output.

FIG. 1 depicts a situation which illustrates one of the challengesencountered in a field application of a ladar sensor of the instantinvention. In this diagram, a vehicle 2 has a long range ladar sensor 4mounted in a headlight assembly at the front of the vehicle. Theassociated illumination pattern 6, is a fan shape shown by the dashedlines. The illumination pattern 6 reflects off the “cateye” typeretro-reflector 12 embedded in the pavement in crosswalk 20, producing areflected ray 14. Cateye retro-reflectors are prismatic reflectors whichare analogous to a corner cube in optics design. Cateye retro-reflectorswere designed in to provide excellent road boundary marking in low lightand blackout conditions. The illumination pattern 6 also reflects off ofthe surface of retro-reflecting stop sign 16, producing reflected ray18. Many stop signs use a retro-reflecting film such as 3M™ DiamondGrade™ reflective sheeting, which is used for traffic control andguidance signs and devices. The film is highly reflective, durable, andmeets a wide variety of sign visibility needs in all light and weatherconditions. Other stop signs use an enhanced film having a prismaticmicrostructure such as 3M™ High Intensity Prismatic (HIP) Sheeting. Thissheeting provides high levels of retro-reflectivity for various trafficscenarios and has excellent long-term reflectivity and durability. Asecond long range ladar sensor 8, with an illumination pattern 10 ismounted in a headlight assembly on the driver side of the vehicle 2.Retro-reflected ray 14 and retro-reflected ray 18 represent very intenseoptical signals which have the potential to saturate the detectors ofthe focal plane array of ladar sensor 4. The intense retro-reflectedrays 14 and 18 may also create optical crosstalk in the focal planearray of ladar sensor 4.

FIG. 2 shows the advantages of the instant invention. The illuminatingpattern 10 of the long range ladar sensor embedded in the driver sideheadlight assembly 8 is now illuminating the cateye retro-reflector 12at an oblique angle. The passenger side ladar sensor 4 has been turnedoff, or is between pulse intervals. The reflection 22 from the cateyeretro-reflector 12 from an oblique illumination will be like an ordinarydiffuse reflector surface and will not have the intense low divergencecharacteristic of the cateye prismatic retro-reflector 12 whenilluminated with light at normal incidence from the passenger side longrange ladar sensor 4 as shown in FIG. 1. In order to produce an accuraterange measurement in the presence of the intense reflections from aretro-reflector 12 positioned normally to the illuminating beam of thelong range ladar sensor 4, the passenger side ladar sensor 4 is operatedin a receive only mode in between pulses which it may send out atregular intervals. The cooperating driver side ladar sensor 8illuminates the scene and the cateye retro-reflector 12. To accuratelygauge the distance to the retro-reflector 12, or to any object in thefield of view, the passenger side ladar sensor must receive anindication of the transmission of an illuminating laser pulse fromdriver side ladar 8. This indication may come in one of two paths, thefirst being a laser flash signal directly from the driver side ladarsensor 8 over bidirectional connections 24. Bidirectional connections 24may be either electrical connections or fiber optic connections,depending on the vehicle design. The second path for indicating thetransmission of an illuminating pulse may come from a central ladarsystem controller 28 over bidirectional connections 26. Either approachmay be used, with similar results once the system is calibrated for thedelays caused by the transmission of the laser flash signal overbidirectional connections 24 or 26. Likewise, the driver side ladarsensor 8 may be operated in a receive only mode, with the sceneilluminating laser pulse coming from the passenger side lasertransmitter in ladar sensor 4. Also shown are a number of short rangeladar sensors 32 mounted at the corners of vehicle 2 in familiar bodypanel cutouts for turn signals, taillights, brake lights, etc. Alsomounted in the same assemblies 32 which house the short range ladarsensors, there may be automotive signal lights, such as turn signal,taillights, brake lights, and visible light or infrared 2D cameras 40.The long range ladar sensors 34 may also share space in the headlightassemblies 4, 8 with visible or infrared 2D cameras 40.

FIG. 3 is a block diagram showing details of a ladar system controller30 and the interconnections with the cooperating systems of a hostvehicle 2. The ladar system controller 30 is an intermediate functionwhich integrates all of the 3D data captured by the various ladarsensors installed on the host vehicle while monitoring the status ofthese sensors, and providing control inputs thereto. The ladar systemcontroller 30 may be subsumed as a piece of software or hardware intothe vehicle CPU 48 in some vehicle designs. The ladar system controller30 transmits commands to the short range ladar sensors SRU1-4 32, and tothe long range ladar sensors LRU1-2 34. A fiber cable and wire harness26 provides the physical media for the transfer of the commands from theladar system controller 30 to the various ladar sensors. 3D data andstatus signals are returned from the various ladar sensors to the ladarsystem controller 30 through fiber cable and wire harness 26. Likewise,command signals are sent to a number (n) of 2D cameras 40, and statusand image data are returned from the 2D cameras via wire harness 26 toladar system controller 30. Both of the long range sensor units 34connect through a set of bidirectional connections which logicallyinclude the transmitters and receivers within each long range sensorunit 34, the physical media of fiber cable and wire harness 26, and thetransmitters and receivers of the ladar system controller 30. Each shortrange sensor unit 32 connects through a set of bidirectional connectionswhich logically include the transmitters and receivers within each shortrange sensor unit, the physical media of fiber cable and wire harness26, and the transmitters and receivers of the ladar system controller30. In some installations, connections directly between ladar sensors 34may be made through bidirectional connections 24. The ladar systemcontroller 30 including D/A and A/D signal converters, may residecompletely or in part on a readout integrated circuit (in FIG. 4). Theladar system controller 30 may have a scene processing capability whichallows it to combine the 3D frames received from each of the operationalladar sensors into a composite 3D map of the entire space directly infront of and surrounding the vehicle 2 and may also merge the 3D mapwith 2D image data received from a number (n) of 2D still or videocameras 40 to provide enhanced resolution, color, and contrast. Theaddition of conventional 2D still or video cameras 40 provide the systemwith enhanced capability for object identification. Ladar systemcontroller 30 receives status data from the ladar sensors indicatinglaser temperature, transmitted laser pulse power and pulse shape,receiver temperature, background light levels, etc. and makes decisionsabout adjustments of global input parameters to the various ladarsensors being controlled. Global settings for detector bias, triggersensitivity, capture modes, filter bandwidth, etc. may be sent fromladar system controller 30 to a given ladar sensor which may overridethe local settings originally set or adjusted by a local controlprocessor residing within a particular ladar sensor. The ladar systemcontroller 30 may also have internal a non-volatile memory to provide astorage location for the programs which run on the ladar systemcontroller 30, and which may be used to store status data and other datauseful at start-up of the system. Residing on ladar system controller 30is a communications port for passing data and control commands andstatus signals over bidirectional connections 42. The communicationsport is typically an Ethernet port or Gigabit Ethernet port, but may bea CAN bus, USB, IEEE1394, Infiniband, or other type data port, and isconnected to provide bidirectional communications with the vehicleelectrical systems and central processing unit 48. Connections 42 may beoptical, electrical, or a combination of both, and include anytransmitters and receivers necessary to condition and transmit the datasignals in both directions. The 3D range data derived from thereflections of the modulated laser light allows for an initial objectmodel to be determined, and for some object identification to take placein a processor of the individual ladar sensors installed on vehicle 2.Refinements of the object model may be made at higher levels in thesystem where data from the several sensors may be integrated with thedata from previous frames. This capability of looking at historical dataas well as current data, allows for some road hazards and collisionthreats to be viewed from a plurality of angles as the vehicle 2 travelsforward, thus eliminating some shadows, while additional shapeinformation is developed from the multiple angles of observation.

Each of the individual ladar sensors may include data processors toreduce the processing load on the ladar system controller 30, thevehicle CPU 48, and the collision processor 44; for example, developingthe point cloud and isolating/segmenting objects in the field of viewand object speed from the point cloud. Conventional 2D visible light orinfrared viewing cameras 40 may be embedded within the ladar sensorsubsystem, and may be part of a sub-assembly containing a ladar sensor.A number (n) of other visible light 2D still or video cameras 56 mayconnect directly to the vehicle collision processor 44 and produce scenedata complementary to the 3D data generated by the various ladar sensorsmounted to the vehicle. The 2D still or video cameras 56 may alsooperate at either visible or infrared wavelengths. Bidirectionalelectrical connections 42 also serve to transfer 3D data maps, status,and control signals between ladar system controller 30 and the vehicleelectrical systems and central processing unit (CPU) 48. At the core ofthe vehicle, an electronic brain may control all functioning of thevehicle 2, and typically controls all other subsystems andco-processors. The electronic brain, or central processing unit (CPU 48)is here lumped together with the basic electrical systems of thevehicle, including battery, headlights, wiring harness, etc. The vehiclesuspension system 46 receives control commands and returns statusthrough bidirectional electrical connections, and is capable ofmodifying the ride height, spring rate, and damping rate of each of thevehicle wheels independently. An inertial reference 50 also has avertical reference, or gravity sensor as an input to the CPU 48. Aglobal positioning reference 54 may also be connected to the vehicle CPU48. The GPS reference 54 may also have a database of all available roadsand conditions in the area which may be updated periodically through awireless link. A duplex radio link 52 may also be connected to CPU 48,and may communicate directly with other vehicles in close range, sharingposition, speed, direction, and vehicle specific information tofacilitate collision avoidance and the free flow of traffic. The duplexradio link may also receive local positional references, road data,weather conditions, and other information important to the operations ofthe vehicle 2 from a central road conditions database through roadsideantennas or cellular stations. The vehicle 2 may also provide vehiclestatus and road conditions updates to the central road conditionsdatabase via radio link 52, allowing the central road conditionsdatabase to be augmented by any vehicle equipped with ladar sensors anda radio link. A collision processor and airbag control unit 44 connectsbidirectionally to CPU 48 as well, receiving inputs from a number ofaccelerometers, brake sensors, wheel rotational sensors, ladar sensors,etc. ACU 44 makes decisions on the timing and deployment of airbags andother restraints. Though the system of FIG. 3 is shown integrated withthe vehicle 2 on which the system is nominally installed, and which isoften an automobile, the system, and any of the described components andsubsystems are designed to be installed on any number of moving vehiclesor stationary platforms.

FIG. 4 is a block diagram of a ladar sensor which describes both longrange ladar sensors 34 and short range sensors 32 typical of thepreferred embodiment. Major improvements described herein include theaddition of a beam steering mechanism 70, an optical gain block 76, anelectronic amplifier array 80, and new detector and focal plane array(FPA) packaging options. The first embodiment provides a 128×128 or128×64 detector array 78 of light detecting elements which is stackedatop a readout integrated circuit 82 using a hybrid assembly method. Inother embodiments of the design, M×N focal plane arrays of lightdetecting elements with M and N having values from 2 to 1024 and greaterare anticipated. The functional elements depicted in FIG. 4 may first bedescribed with respect to the elements of a typical long range ladarsensor 34. A control processor 58 controls the functions of the majorcomponents of the ladar sensor 34. Control processor 58 connects topulsed laser transmitter 68 through bidirectional electrical connections(with interface logic, analog to digital (A/D) and digital to analog(D/A) converters 66) which transfer commands from control processor 58to pulsed laser transmitter 68 and return monitoring signals from pulsedlaser transmitter 68 to the control processor 58. The interface logic,including analog to digital (A/D) and digital to analog (D/A) converters66, may reside completely or in part on an integrated circuit. A lightsensitive diode detector (Flash Detector) 67 is placed at the back facetof the laser so as to intercept a portion of the laser light pulseproduced by the pulsed laser transmitter 68. An optical sample of theoutbound laser pulse taken by an optical sampler from the front facet ofpulsed laser transmitter 68 is routed to a corner of the detector array78 as an automatic range correction (ARC) signal, typically over a fiberoptic cable. The pulsed laser transmitter 68 may be a solid-state laser,monoblock laser, semiconductor laser, fiber laser, or an array ofsemiconductor lasers. It may also employ more than one individual laserto increase the data rate. In a preferred embodiment, pulsed lasertransmitter 68 is an array of vertical cavity surface emitting lasers(VCSELs). In an alternative embodiment, pulsed laser transmitter 68 is adisc shaped solid state laser of erbium doped phosphate glass pumped by976 nanometer semiconductor laser light.

In operation, the control processor 58 initiates a laser illuminatingpulse by sending a logic command or modulation signal to pulsed lasertransmitter 68, which responds by transmitting an intense burst of laserlight through beam steering mechanism 70 and transmit optics 72. In thecase of a Q-switched solid state laser based on erbium glass,neodymium-YAG, or other solid-state gain medium, a simple bi-level logiccommand may start the pump laser diodes emitting into the gain mediumfor a period of time which will eventually result in a single flash ofthe pulsed laser transmitter 68. In the case of a semiconductor laserwhich is electrically pumped, and may be modulated instantaneously bymodulation of the current signal injected into the laser diode, amodulation signal of a more general nature is possible, and may be usedwith major beneficial effect. The modulation signal may be a flat-toppedsquare or trapezoidal pulse, or a Gaussian pulse, or a sequence ofpulses. The modulation signal may also be a sinewave, gated or pulsedsinewave, chirped sinewave, or a frequency modulated sinewave, or anamplitude modulated sinewave, or a pulse width modulated series ofpulses. The modulation signal is typically stored in memory 64 as alookup table of digital memory words representative of analog values,which lookup table is read out in sequence by control processor 58 andconverted to analog values by an onboard digital-to-analog (D/A)converter 66, and passed to the pulsed laser transmitter 68 drivercircuit. The combination of a lookup table stored in memory 64 and a D/Aconverter, along with the necessary logic circuits, clocks, and timers62 resident on control processor 58, together comprise an arbitrarywaveform generator (AWG) circuit block. The AWG circuit block mayalternatively be embedded within a laser driver as a part of pulsedlaser transmitter 68. Transmit optics 72 diffuse the high intensity spotproduced by pulsed laser transmitter 68 substantially uniformly over thedesired field of view to be imaged by the ladar sensor 34. An opticalsample of the transmitted laser pulse (termed an ARC signal) is alsosent to the detector array 78 via optical fiber. A few pixels in acorner of detector array 78 are illuminated with the ARC (AutomaticRange Correction) signal, which establishes a zero time reference forthe timing circuits in the readout integrated circuit (ROIC) 82. Eachunit cell of the readout integrated circuit 82 has an associated timingcircuit which is started counting by an electrical pulse derived fromthe ARC signal. Alternatively, the flash detector 67 signal may be usedas a zero reference in a second timing mode. Though the ARC signalneatly removes some of the variable delays associated with transit timethrough the detector array 78, additional cost and complexity is theresult. Given digital representations of the image frames, the same taskmay be handled in software/firmware by a capable embedded processor suchas data reduction processor 86. When some portion of the transmittedlaser pulse is reflected from a feature in the scene in the field ofview of the ladar sensor 34, it may be incident upon receive optics 74,typically comprising the lens of a headlamp assembly and an array ofmicrolenses atop detector array 78. Alternative embodiments use enhanceddetectors which may not require the use of microlenses. Otheralternative embodiments of receive optics 74 employ diffractive arraysto collect and channel the incoming light to the detector array 78individual elements. Pulsed laser light reflected from a feature in thescene in the field of view of receive optics 74 is focussed onto anindividual detector element of the detector array 78. This reflectedlaser light optical signal is then detected by the affected detectorelement and converted into an electrical current pulse which is thenamplified by an associated pixel amplifier circuit of optional amplifierarray 80, and the unit cell electrical circuit of the readout integratedcircuit 82, and the time of flight measured. Thus, the range to eachreflective feature in the scene in the field of view is measurable bythe ladar sensor 34. The detector array 78, amplifier array 80, andreadout integrated circuit 82 may be an M×N or N×M sized array. Controlprocessor 58 directs the beam steering mechanism 70 to deflect theoutput beam of the pulsed laser transmitter 68 into a selected portionof the scene in the forward path of vehicle 2, as illustrated in FIG. 1.

Continuing with FIG. 4, receive optics 74 may be a convex lens,spherical lens, cylindrical lens or diffractive grating array. Anoptional mechanical shutter may be used by control processor 58 tocalibrate the system or protect the detector array 78. This capabilty isdescribed in detail in association with FIG. 9. Receive optics 74collect the light reflected from the scene and focus the collected lighton the detector array 78. In a preferred embodiment, detector array 78is formed in a thin film of gallium arsenide deposited epitaxially atopan indium phosphide semiconducting substrate. Typically, detector array78 would have a set of cathode contacts exposed to the light and a setof anode contacts electrically connected to the supporting amplifierarray 80 and readout integrated circuit 82 through a number of indiumbumps deposited on the detector array 78. The cathode contacts of theindividual detectors of detector array 78 are then connected to adetector bias voltage grid on the illuminated side of the array. Eachanode contact of the detector elements of detector array 78 is thusindependently connected to an input of a pixel amplifier of optionalamplifier array 80, or directly to a unit cell electronic circuit ofreadout integrated circuit 82. This traditional hybrid assembly ofdetector array 78 and readout integrated circuit 82 may still be used,but new technology may reduce inter-element coupling, or crosstalk, andreduce leakage (dark) current and improve efficiency of the individualdetector elements of detector array 78. Other detector array structuresare developed herein and described in association with FIGS. 11, 15, and16. Readout integrated circuit 82 comprises a rectangular array of unitcell electrical circuits. Each unit cell has the capability ofamplifying a low level photocurrent received from an optoelectronicdetector element of detector array 78, and sampling the amplifieroutput. Typically the unit cell is also capable of detecting thepresence of an electrical pulse in the pixel amplifier output associatedwith a light pulse reflected from the scene and intercepted by thedetector element of detector array 78. The detector array 78 may be anarray of avalanche photodiodes, capable of photoelectron amplification,and modulated by an incident light signal at the design wavelength. Thedetector array 78 elements may also be a P-intrinsic-N (PIN) photodiodesor N-intrinsic-P (NIP) photodidodes with the dominant carrier beingholes or electrons respectively. In the case of an NIP detectorstructure, the corresponding ROIC 82 would have the polarity of the biasvoltages and amplifier inputs adjusted accordingly. The hybrid assemblyof detector array 78 and readout integrated circuit 82 of the preferredembodiment is shown in FIGS. 11-17, and the assembly is then mounted toa supporting circuit assembly, typically on a FR-4 substrate or ceramicsubstrate. The circuit assembly typically provides support circuitrywhich supplies conditioned power, a reference clock signal, calibrationconstants, and selection inputs for the readout column and row, amongother support functions, while receiving and registering range andintensity outputs from the readout integrated circuit 82 for theindividual elements of the detector array 78. Many of these supportfunctions may be implemented in Reduced Instruction Set Computer (RISC)processors which reside on the same circuit substrate. A detector biasconverter circuit 96 applies a time varying detector bias to thedetector array 78 which provides optimum detector bias levels to reducethe hazards of saturation in the near field of view of detector array78, while maximizing the potential for detection of distant objects inthe field of view of detector array 78. The contour of the time varyingdetector bias supplied by detector bias converter 96 is formulated bycontrol processor 58 based on feedback from the data reduction processor86, indicating the reflectivity and distance of objects or points in thescene in the field of view of the detector array 78. Control processor58 also provides several clock and timing signals from a timing core 62to readout integrated circuit 82, data reduction processor 86,analog-to-digital converters 84, object tracking processor 98, and theirassociated memories. Control processor 58 relies on a temperaturestabilized or temperature compensated frequency reference 94 to generatea variety of clocks and timing signals. Temperature stabilized frequencyreference 94 may be a temperature compensated crystal oscillator (TCXO),dielectric resonator oscillator (DRO), or surface acoustic wave device(SAW). Timing core 62 resident on control processor 58 may include ahigh frequency tunable oscillator, programmable prescaler dividers,phase comparators, and error amplifiers.

Continuing with FIG. 4, control processor 58, data reduction processor86, and object tracking processor 98 each have an associated memory forstoring programs, data, constants, and the results of operations andcalculations. These memories, each associated with a companion digitalprocessor, may include ROM, EPROM, or other non-volatile memory such asflash. They may also include a volatile memory such as SRAM or DRAM, andboth volatile and non volatile memory may be integrated into each of therespective processors. A common frame memory 88 serves to hold a numberof frames, each frame being the image resulting from a single laserpulse. Both data reduction processor 86 and object tracking processor 98may perform 3D image processing, to reduce the load on a sceneprocessing unit normally associated with a higher level processor, forexample ladar system controller 30. There are two modes of datacollection, the first being SULAR, or a progressive scan in depth. Eachlaser pulse typically results in 20 “slices” of data, similar to a CATscan, and each “slice” may be stored as a single page in the commonframe memory 88. With each pixel sampling at a 2 nanosecond interval,the “slices” are each a layer of the image space at roughly 1 footdifferences in depth. The 20 slices represent a frame of data, and thesampling for a succeeding laser pulse may be started at 20 feet furtherin depth, so that the entire image space up to 1000 feet in range ordepth, may be swept out in a succession of 50 laser illuminating pulses,each laser pulse response consisting of 20 “slices” of data held in asingle frame entry. In some cases, the frame memory may be large enoughto hold all 50 frames of data. The number of slices stored could beenough to map out any relevant distance, with no trigger mode operationrequired. The reduction of the data might then take place in an externalcomputer, as in the case of data taken to map an underwater surface, ora forest with tree cover, or any static landscape, where sophisticatedpost-processing techniques in software may yield superior accuracy orresolution. A second data acquisition mode is the TRIGGER mode, wherethe individual pixels each look for a pulse response, and upon a certainpulse threshold criteria being met, the 20 analog samples bracketing thepulse time of arrival are retained in the pixel analog memories, and arunning digital counter is frozen with a nominal range measurement. The20 analog samples are output from each pixel through the “A” and “B”outputs of readout integrated circuit 82, which represent theinterleaved row or column values of the 128×128 pixel of the presentdesign. The “A” and “B” outputs are analog outputs, and the analogsamples presented there are converted to digital values by the dualchannel analog-to-digital (A/D) converter 84. Interleaving the outputsmeans one of the outputs (“A”) reads out the odd numbered lines of thereadout IC 82, and the other output (“B”) reads out the even numberedlines of the readout IC 82. Larger detector arrays 78 and readout ICs 82may have more than two analog outputs. The digital outputs of the A/Dconverters 84 connect to the inputs of the data reduction processor 86.A/D converters 84 may also be integrated into readout integrated circuit82. The digital outputs are typically 10 or 12 bit digitalrepresentations of the uncorrected analog samples measured at each pixelof the readout IC 82, but other representations with greater or fewerbits may be used, depending on the application. The rate of the digitaloutputs depends upon the frame rate and number of pixels in the array.In TRIGGER mode, a great deal of data reduction has already transpired,since the entire range or depth space may be swept out in the timeframeof a single laser pulse, and the data reduction processor 86 would onlyoperate on the 20 analog samples stored in each unit cell in order torefine the nominal range measurement received from each pixel (unitcell) of the array. The data reduction processor 86 refines the nominalrange measurements received from each pixel by curve fitting of theanalog samples to the shape of the outgoing laser illuminating pulse,which is preserved by the reference ARC pulse signal. These pulses aretypically Gaussian, but may be square, trapezoidal, haversine, sincfunction, etc., and the fitting algorithms may employ Fourier analysis,Least Squares analysis, or fitting to polynomials, exponentials, etc.The range measurements may also be refined by curve fitting to a wellknown reference pulse characteristic shape. In TRIGGER acquisition mode,the frame memory 88 only needs to hold a “point cloud” image for eachilluminating laser pulse. The term “point cloud” refers to an imagecreated by the range and intensity of the reflected light pulse asdetected by each pixel of the 128×128 array of the present design. InTRIGGER mode, the data reduction processor 86 serves mostly to refinethe range and intensity (R&I) measurements made by each pixel prior topassing the R&I data to the frame memory 88 over data bus 87, and no“slice” data or analog samples are retained in memory independently ofthe R&I “point cloud” data in this acquisition mode. Frame memory 88provides individual or multiple frames, or full point cloud images, tocontrol processor 58 over data bus 90, and to an optional objecttracking processor 98 over data bus 89 as required.

As also shown in FIG. 4, data reduction processor 86 and controlprocessor 58 may be of the same type, a reduced instruction set (RISC)digital processor with hardware implementation of integer and floatingpoint arithmetic units. Object tracking processor 98 may also be of thesame type as RISC processors 86 and 58, but may in some cases be aprocessor with greater capability, suitable for highly complex graphicalprocessing. Object tracking processor 98 may have in addition tohardware implemented integer and floating point arithmetic units, anumber of hardware implemented matrix arithmetic functions, includingbut not limited to; matrix determinant, matrix multiplication, andmatrix inversion. In operation, the control processor 58 controlsamplifier array 80, readout integrated circuit 82, A/D converters 84,frame memory 88, data reduction processor 86 and object trackingprocessor 98 through a bidirectional control bus 92 which allows for themaster, control processor 58 to pass commands on a priority basis to thedependent peripheral functions; amplifier array 80, readout IC 82, A/Dconverters 84, frame memory 88, data reduction processor 86, and objecttracking processor 98. Bidirectional control bus 92 also serves toreturn status and process parameter data to control processor 58 fromamplifier array 80, readout IC 82, A/D converters 84, frame memory 88,data reduction processor 86, and object tracking processor 98. Datareduction processor 86 refines the nominal range data and adjusts eachpixel intensity data developed from the digitized analog samplesreceived from A/D converters 84, and outputs a full image frame viaunidirectional data bus 87 to frame memory 88, which is a dual portmemory having the capacity of holding several frames to severalthousands of frames, depending on the application. Object trackingprocessor 98 has internal memory with sufficient capacity to holdmultiple frames of image data, allowing for multi-frame synthesisprocesses, including video compression, single frame or multi-frameresolution enhancement, statistical processing, and objectidentification and tracking. The outputs of object tracking processor 98are transmitted through unidirectional data bus 99 to a communicationsport 60, which may be resident on control processor 58. All slice data,range and intensity data, control, and communications then pass betweencommunications port 60 and a centralized ladar system controller 30,(FIG. 3) through bidirectional connections 26. Power and groundconnections (not shown) may be supplied through an electromechanicalinterface. Bidirectional connections 26 may be electrical or opticaltransmission lines, and the electromechanical interface may be a DB-25electrical connector, or a hybrid optical and electrical connector, or aspecial automotive connector configured to carry signals bidirectionallyfor the ladar sensor 34. Bidirectional connections 26 also would connectto ladar system controller 30 in the case of an auxiliary lamp assemblywhich may have a short range ladar sensor 32 embedded therein.Bidirectional connections 26 may be high speed serial connections suchas Ethernet, Universal Serial Bus (USB), or Fibre Channel, or may alsobe parallel high speed connections such as Infiniband, etc., or may be acombination of high speed serial and parallel connections, withoutlimitation to those listed here. Bidirectional connections 26 also serveto upload information to control processor 58, including program updatesfor data reduction processor 86, object tracking processor 98, andglobal position reference data, as well as application specific controlparameters for the remainder of the ladar sensor 34 functional blocks.Inertial and vertical reference 50 (see FIG. 3) also provides data tothe short range ladar sensors 32 and long range ladar sensors 34 fromthe host vehicle 2 through the vehicle electrical systems and CPU 48 andthe ladar system controller 30 as needed. Likewise, any other data fromthe host vehicle 2 which may be useful to the ladar sensor 34 may beprovided in the same manner as the inertial and vertical reference data.Inertial and vertical reference data may be utilized in addition toexternal position references by control processor 58, which may passposition and inertial reference data to data reduction processor 86 foradjustment of range and intensity data, and to object tracking processor98 for utilization in multi-frame data synthesis processes. The verticalreference commonly provides for measurement of pitch and roll, and isadapted to readout an elevation angle, and a twist angle (analogous toroll) with respect to a horizontal plane surface normal to the force ofgravity. The short range ladar sensors 32 typically employ asemiconductor laser, which may be modulated in several different ways.The long range ladar sensors 34 typically employ a q-switched solidstate laser, which produces a single output pulse with a Gaussianprofile. The pulse shape of a solid state laser of this type is noteasily modulated, and therefore must be dealt with “as is” by thereceiver section of a long range ladar sensor 34. The operations ofshort range ladar sensors 32 of the type which are typically embedded inan auxiliary lamp assembly such as a taillight, turn signal, or parkinglight are the same as the operations of the long range ladar sensors 34with some exceptions. The long range ladar sensors 34 and short rangeladar sensors 32 may differ only in the type of laser employed and thetype of laser modulation. The transmit optics 72 and receive optics 74may also differ, owing to the narrower angular field of view for thelong range ladar sensors 34. Differences in the transmitted laser pulsemodulation between the long range ladar sensors 34 and short range ladarsensors 32 may be accommodated by the flexible nature of the readout IC82 sampling modes, and the data reduction processor 86 programmability.The host vehicle 2 may have a number of connector receptacles generallyavailable for receiving mating connector plugs from USB, Ethernet,RJ-45, or other interface connection, and which may alternatively beused to attach long range ladar sensors 34 or short range ladar sensors32 of the type described herein.

Continuing with FIG. 4, it is useful to discuss a short range ladarsensor 32 variant. In a short range ladar sensor 32, considerably lesstransmit power is required, allowing for the use of a semiconductorlaser and multi-pulse modulation schemes. One example of a semiconductorlaser is the vertical cavity surface emitting laser (VCSEL), used in apreferred embodiment because of a number of preferentialcharacteristics. A VCSEL typically has a circular beam profile, and haslower peak power densities at the aperture. VCSELs also require fewersecondary mechanical operations, such as cleaving, polishing, etc., andmay be formed into arrays quite easily. The use of a semiconductor laserallows for the tailoring of a drive current pulse so as to produce aGaussian optical pulse shape with only slight deviations. The VCSELresponse time is in the sub-nanosecond regime, and the typical pulseoptical width might be 5-100 nanoseconds at the half power points. Inthe diagram of FIG. 4, the VCSEL and laser driver would be part of thepulsed laser transmitter 68, and the desired pulse or waveshape isitself produced by a digital-to-analog converter 66 which has a typicalconversion rate of 200-300 MHz, so any deviations in the output pulseshape from the Gaussian ideal may be compensated for in the lookup tablein memory 64 associated with control processor 58, which serves as thedigital reference for the drive current waveform supplied to the laserdriver within pulsed laser transmitter 68 by the D/A converter. AGaussian single pulse modulation scheme works well at short ranges,given the limited optical power available from a VCSEL. Extending therange of a VCSEL transmitter may be done using more sophisticatedmodulation schemes such as multi-pulse sequences, sinewave bursts, etc.The VCSEL and modulation schemes as described herein with reference toshort range ladar sensor 32 are an alternative to the solid state lasertypically used in a pulsed laser transmitter 68 of a long range ladarsensor 34. The use of a VCSEL array in pulsed laser transmitter 68 hasthe potential to reduce cost, size, power consumption, and/or enhancereliability. Ladar sensors may be mounted at many points on the vehicle2; headlamps, auxiliary lamps, door panels, rear view mirrors, bumpers,etc. When equipped with a more sensitive detector array 78 such as animage tube FPA, a ladar sensor of the type described herein may use aVCSEL array as an illuminating source, and much longer ranges may besupported. When referring to the major functions of the ladar sensor ofFIG. 4, it is sometimes convenient to refer to the “optical transmitter”as those functions which support and/or create the burst of light forilluminating the scene in the field of view. These elements wouldtypically be the control processor 58 which starts the process, pulsedlaser transmitter 68, beam steering mechanism 70, and transmit optics72. The term “optical receiver” may be used to refer to those elementsnecessary to collect the light reflected from the scene in the field ofview, filter the received light, convert the received light into aplurality of pixellated electrical signals, amplify these pixellatedelectrical signals, detect the pulses or modulation thereon, perform therange measurements, and refine or reduce the received data. Thesefunctions would include the receive optics 74, optical gain block 76,detector array 78, amplifier array 80, readout IC 82, A/D converters 84,and the data reduction processor 86.

The unit cell electronics depicted in FIG. 5 is well adapted to workwith a Gaussian single pulse modulation scheme, and works advantageouslywith other modulation schemes as well, including sequences offlat-topped pulses, Gaussian, or otherwise shaped pulses. These pulsesmay be of varying width and spacing, in order to reduce rangeambiguities, and may also be random pulse sequences, or in other cases,Barker coded pulse sequences. In the typical operation of a short rangeladar sensor 32 having a semiconductor laser producing a single Gaussianoutput pulse, some portion of the pulsed laser light reflected from asurface in the field of view of the short range ladar sensor 32 isconcentrated and focused by receive optics 74, passes through opticalgain block 76, and falls on an individual detector element 100 ofdetector array 78. The individual element 100 is typically an avalanchephotodiode, but may be a PIN or NIP, or other structure. Each individualelement 100 of detector array 78 is formed in a semiconducting filmcomprised of silicon, indium gallium arsenide, indium gallium arsenidephosphide, aluminum gallium arsenide, indium gallium nitride, or othersemiconducting compound appropriate to the wavelength of operation. Eachindividual element 100 is biased with a voltage by a bias voltagedistribution network V_(DET) 102. The reflected light signal incidentupon the individual detector element 100 is converted to an electronicsignal, typically a photocurrent, and amplified by input amplifier 104,typically a transimpedance amplifier. The output of input amplifier 104is distributed to a trigger circuit 106 as well as a number of analogsampling gates 108. Each analog sampling gate 108 has an outputconnected to an analog memory cell 120. The trigger circuit 106 istypically a threshold voltage comparator, set to trigger when a pulse isreceived which exceeds a predetermined magnitude, though other pulsedetection schemes may be used. After a programmable delay through delaycircuit 110, the state of circular selector 112 is frozen by the logictransition of trigger circuit 106 output if the unit cell is beingoperated in TRIGGER mode. Prior to the detection of a received pulse bytrigger circuit 106, the sample clock 114 causes the state of circularselector 112 to advance, enabling one of the sampling control outputsS1-S3, which in turn causes a sampling of the input amplifier 104 outputby one of the sampling gates 108. The number of transitions of sampleclock 114 is counted by counter 116, as the circular selector 112outputs a logic transition to counter 116 for every cycle of thesampling clock after the release of the active low reset line 118.Circular selector 112 may cycle through outputs S1-S3 in order, or mayhave a different sequence, depending on the programming. A secondcircular selector 112, and sample clock 114 may operate in parallel,along with counter 116, analog sampling gates 108 and analog memorycells 120. The combination of sample clock 114, counter 116, circularselector 112, sampling gates 108, and memory cells 120 may be termed aunit cell sampling structure 122, indicated by the short dashed lineborder. Two, three, or more of these sampling structures 122 may beoperated in parallel on the output of input amplifier 104, with theadvantages of such a structure to be described later in regards to rangeambiguity. Shown in FIG. 5 are three sampling gates 108, and analogmemory cells 120, but the number may be several hundred or more on somereadout ICs 82. Once all of the analog sample data has been taken, acontrol command from the control processor 58 initiates a readout cycleby activating output control 124 and output amplifier 126 to readout thecontents of the analog memory cells 120 in a predetermined order.

In a typical short range ladar sensor 32, and assuming a 1 cm² VCSELarray with a 5 kW/cm² power density, and depending upon the reflectivityof the objects in the field of view, and the responsivity and excessnoise of the detector array 78, the effective range of a Gaussian singlepulse modulation scheme might be in the range of 10-20 meters, using asimple threshold detection technique. Without resorting to a large VCSELarray, which might be expensive and might require a large dischargecapacitor to supply a large current pulse, more sophisticated modulationand detection techniques can be used to create additional processinggains, to effectively increase the signal-to-noise ratio, and thusextend the range of the short range ladar sensor 32 without requiring anincrease in peak power. In a first modulation scheme, which produces aGaussian single pulse modulation, a detection technique may be employedwhich uses the digitized analog samples from each unit cell electricalcircuit, and processes these samples in a digital matched filter to findthe centroid of the received pulse, resulting in significant processinggain. The processing gains resulting from this structure areproportional to the square root of the number of samples used in thefiltering algorithm. For example, a unit cell electrical circuit with256 analog memory cells 120 could yield a processing gain of 16 if allthe available analog samples were used in a matched filter algorithm,assuming Gaussian single pulse modulation, and a normal noisedistribution. The term “processing gain” is used here to describe theincrease in effective signal-to-noise ratio (SNR) realized by performingthe described operations on the voltage samples. Assuming the pulsedlaser light is distributed uniformly over just the field of view of thereceive optics 74, the effective range of the ladar also increases asthe square root of the transmitted power (or SNR), and an increase inrange to 40-80 meters could be the result. Single pulse Gaussianmodulation may be characteristic of either a solid state laser or asemiconductor laser with a simple driver, and thus may be an attributeof either a long range ladar sensor 34 or a short range ladar sensor 32.

The unit cell electronic circuit of FIG. 5 is well adapted to singlepulse modulation, or to more complex modulation scenarios. In a secondmodulation scheme, a VCSEL array modulated with a series of Barkerencoded flat-topped or Gaussian pulses can be sampled by the unit cellelectronics of FIG. 5 and analyzed by data reduction processor 86 forrange and intensity estimates. In a third modulation scheme, a VCSELarray modulated with a pulsed sinewave allows for greater cumulativeenergy to be reflected from a feature in a scene in the field of view ofeither a short range ladar sensor 32 or a long range ladar sensor 34without an increase in peak power. Each peak of a pulsed sinewave willhave a separate reflection from an object or feature in the scene in thefield of view of the ladar sensor 52 and the unit cell electricalcircuit of FIG. 5 allows the ladar sensor receiver to respond to thecumulative energy from many of these reflected pulses using a minimum ofcircuitry. The waveform in a preferred embodiment is a number ofsinewave cycles, and the number could be quite large, depending on anumber of factors. The receiver circuitry of the unit cell electronicsshown in FIG. 5 is capable of sampling or of synchronously detecting thecumulative energy of the returned pulse peaks. Two sampling modes may besupported by the unit cell sampling structure shown in FIG. 5. Whentaking analog samples of single pulse or multi pulse sequences, whereinanalog samples of an incoming waveform are being sequentially taken, thesampling impedance control 128 (Z) to the circular selector 112 would beset to a minimum value. The sampling frequency of sample clock 114 wouldalso be selected to produce 10 or perhaps 20, analog samples during eachpulse width. When the sampling impedance control 128 is set to aminimum, the sample controls S1, S2, S3 turn on with full voltage duringa sampling cycle. Since each sampling gate 108 is a field effecttransistor, increasing the sample control voltage S1-S3 will increasethe gate-source voltage on the sampling FET, thus lowering the impedanceof the channel between source and drain, and setting the sampling gateimpedance to a minimum. When the sampling gate 108 impedance is set to aminimum, the storage capacitor serving as analog memory cell 120 chargesrapidly to the voltage present at the output of input amplifier 104.This mode can be termed “instantaneous voltage sampling” to distinguishthe mode from a second sampling mode, which is selected when thesampling impedance control 128 is set to a higher, or even maximumvalue. When the sampling impedance control 128 is selected for highimpedance, or maximum series resistance value, the outputs S1-S3 wouldbe at or near minimum voltages when enabled, resulting in a lowergate-source voltage across each of the sampling gate FETs 108, and thusa higher sampling gate series resistance in the channel between sourceand drain of each sampling gate 108 FET. With the series resistance ofthe sampling gates 108 set to high or maximum value, the effect is tocause an R-C filter to develop, with the analog memory cell 120 storagecapacitor performing as an integrating capacitor. This second samplingmode may be very useful when a sinusoidal modulation is applied to thepulsed laser transmitter 68 in the case where the laser is asemiconductor laser, typically a high efficiency VCSEL. By applying asampling clock to the sampling gate 108 driven by S1, and which is thesame frequency as the sinusoidal modulation, a sum frequency and adifference frequency will be in the sampled signal, and the analogmemory cell 120 storage capacitor will filter out the sum frequency, andthe difference frequency will be zero, leaving only a DC voltagecomponent, which will be a trigonometric function of the phasedifference. Over a number of cycles of the sinusoidal modulation fromthe output of input amplifier 104, this DC voltage will emerge as thesine or cosine of the phase difference between the transmitted andreceived waveforms. This phase difference is proportional to the rangeto a reflecting surface. To improve the processing gain, the secondsampling gate driven by the S2 signal is driven by the same samplingclock frequency, but shifted by 90 degrees in phase, and the greater ofthe two DC voltages, or a ratio of the two voltages, may be used toestimate phase, and thereby range. Typically, a ratio is preferred, asit removes the variation in amplitude of the incoming sinewave as anerror term. This type of detection relies on “In-phase” and“Quadrature-phase” local references, and is often referred to as an“I&Q” detection scheme. Thus, the sampling gates 108 can be operated asinstantaneous voltage samplers in a first sampling mode, or as frequencymixers in a second sampling mode, depending on the state of the samplingimpedance control 128, and the frequency applied by sampling clock 114.In the first sampling mode, the shape of a pulse or sequence of pulsesmay be acquired, and in second sampling mode, a periodic waveformmodulation such as a sinewave, may be demodulated through the frequencymixing effect and integration on a storage capacitor, resulting in aphase measurement and thereby range. Demodulation within the unit cellelectrical circuit reduces the data at an early point, reducing therequirements for memory and fast digital processors. Alternatively, thedemodulation of a sinewave or other periodic waveform may be performedin data reduction processor 86 on the digitized representations of theanalog samples, given a fast arithmetic unit, and the proper algorithm.This illustrates the power and flexibility of the instantaneous voltagesampling mode, as the data reduction processor 86 can be adapted to runPWD, CSC, FIR filter, IIR filter, I&Q, or any number of curve fittingalgorithms to increase SNR, measure phase, or otherwise reduce rangemeasurement errors.

FIG. 6A shows a central section view of a preferred embodiment of thepulsed laser transmitter 68, flash detector 67, and beam steeringmechanism 70. A silicon substrate 130 is processed via photolithographyand MEMs fabrication techniques to have a cavity 142, cantileveredbending element 138, and two recessed sections for mounting an edgeemitting laser 132, and a beam conditioning lens 134. Beam conditioninglens 134 is typically a ball lens or a rod lens, but may be of anothertype useful for circularizing and/or collimating the elliptical outputbeam of edge emitting laser 132. An angle block 136 is used to redirectthe conditioned output beam of edge emitting laser 132 onto the mirrorsurface 146 of bending element 138. Mirror surface 146 may be gold,silver, aluminum, nickel, titanium, or other reflective metal, and maybe a stack of several metal films. Mirror surface 146 may also be amulti layer dielectric reflector formed of silicon dioxide, siliconnitride, sapphire, calcium fluoride, or other suitable dielectric film.In operation, a differential voltage is applied between interdigitalcontacts 148 to cause piezoelectric strain on the reverse side ofbending element 138. If the piezoelectric strain is compressive, thebending element 138 will bend up (above the top surface of substrate130). If the piezoelectric strain is tensile, the bending element 138will deflect down towards the interior of cavity 142. Depending on thesign of the differential voltage imposed on interdigital contacts 148,the strain will be compressive or tensile. In this manner, the outputoptical beam 140 may be swept through an angle θ as shown in thediagram. The angle θ is typically in the range of 17 degrees, and may beswept out in less than a millisecond, and in some cases, less than 100microseconds, depending on the particulars of the design. At the rear ofedge emitting laser 132 is an edge detecting PIN diode 144 placed as anoptical power detector to intercept the fractional power transmittedthrough the back facet of edge emitting laser 132. The pulsed lasertransmitter 68 has a number of other components besides laser 132,including an electrical drive circuit, and electronic interface to thecontrol processor 58. These circuits are not shown here, but have beendescribed in other publications. Likewise, the flash detector 67 hasother components than PIN diode 144, including an electrical amplifierand threshold detection circuit. These circuits have been described indetail in other publications as well, and are peripheral to the instantinvention.

FIG. 6B shows a central section view of a second embodiment of beamsteering mechanism 70. Several embodiments of the beam steeringmechanism 70 are anticipated, and each is described in association witha FIGS. 6A, 6B, 6C, and 6D. The operating mode of the beam steeringmechanism 70 will be then described in connection with the operations ofthe ladar sensor (32, 34). A silicon substrate 150 is processed viaphotolithography and MEMs fabrication techniques to have a cavity 156,cantilevered bending element 158, and two recessed sections for mountingan edge emitting laser 152, and a beam conditioning lens 154 on a firstsurface. Beam conditioning lens 154 is typically a ball lens or a rodlens, but may be of another type useful for circularizing and/orcollimating the elliptical output beam of edge emitting laser 152. Theoutput beam of edge emitting laser 152 is directed by lens 154 to fallon the mirror surface 160 of bending element 158. Mirror surface 160 maybe gold, silver, aluminum, nickel, titanium, or other reflective metal,and may be a stack of several metal films. Mirror surface 160 may alsobe a multi layer dielectric reflector formed of silicon dioxide, siliconnitride, sapphire, calcium fluoride, or other suitable dielectric film.In operation, a differential voltage is applied between interdigitalcontacts 162 to cause piezoelectric strain on the reverse side ofbending element 158. If the piezoelectric strain is tensile, the bendingelement 158 will bend up (above the top surface of substrate 150). Ifthe piezoelectric strain is compressive, the bending element 158 willdeflect down towards the interior of cavity 156. Depending on the signof the differential voltage imposed on interdigital contacts 162, thestrain will be compressive or tensile. In this manner, the outputoptical beam 164 may be swept through an angle θ as shown in thediagram. The angle θ is typically in the range of 17 degrees, and may beswept out in less than a millisecond, and in some cases, less than 100microseconds, depending on the particulars of the design. Acomplementary structure may be formed on the reverse side of substrate150 in the same manner, and the identical structure used in combinationwith the structure on the first surface to sweep out an angle 2θ whichwould then be in the range of 34 degrees. In some cases, the structureon the reverse side of substrate 150 is formed by assembling togethertwo similar MEMs substrates 150 and 151, by wafer bonding or otherprocesses. A knit line 166 is shown where the two substrates 150 and 151are joined.

FIG. 6C shows a central section view of a third embodiment of beamsteering mechanism 70. A first silicon substrate 168 is processed viaphotolithography and MEMs fabrication techniques to have a cavity 178,and a first cantilevered bending element 180, and two recessed sectionsfor mounting an edge emitting laser 170, and a beam conditioning lens172 on the top surface. A third recessed section 175 is formed by MEMstechniques, such as chemical mechanical polishing. Recessed section 175has an angled profile on the sidewalls, and a mirrored surface 174. Beamconditioning lens 172 is offset vertically from the centerline of theoutput beam of laser 170, and redirects the beam onto mirrored surface174. Beam conditioning lens 172 is typically a ball lens or a rod lens,but may be of another type useful for circularizing and/or collimatingthe elliptical output beam of edge emitting laser 170. The optical beamis then directed against the mirror surface 190 of a second bendingelement 188. Second bending element 188 is formed in a second siliconsubstrate 184 which has a cavity 186 formed therein by MEMS processes.Second silicon substrate 184 is positioned atop first silicon substrate168 and bonded. The bonding process may involve adhesive, epoxy, orother chemical/physical means of surface activation. Mirror surface 190may be gold, silver, aluminum, nickel, titanium, or other reflectivemetal, and may be a stack of several metal films. Mirror surface 190 mayalso be a multi layer dielectric reflector formed of silicon dioxide,silicon nitride, sapphire, calcium fluoride, or other suitabledielectric film. In operation, a differential voltage is applied betweeninterdigital contacts 192 to cause piezoelectric strain on the reverseside of second bending element 188. If the piezoelectric strain iscompressive, the bending element 188 will bend up (above the top surfaceof second silicon substrate 184). If the piezoelectric strain istensile, the bending element 188 will deflect down towards the interiorof cavity 186. Depending on the sign of the differential voltage imposedon interdigital contacts 192, the strain will be compressive or tensile.In this manner, the output optical beam 194 may be swept through anangle of about 17 degrees as shown in the diagram. The output beamdirected by second bending element 188 is then incident upon themirrored surface 182 of first bending element 180. In operation, adifferential voltage is applied between interdigital contacts 176 tocause piezoelectric strain on the reverse side of first bending element180. If the piezoelectric strain is tensile, the bending element 180will bend up (above the top surface of silicon substrate 168). If thepiezoelectric strain is compressive, the bending element 180 willdeflect down towards the interior of cavity 178. Depending on the signof the differential voltage imposed on interdigital contacts 176, thestrain will be compressive or tensile. In this manner, the outputoptical beam 194 may be swept through an angle θ in the range of 34degrees, due to the cascade of the two MEMs bending elements. The angleθ may be swept out in less than a millisecond, and in some cases, lessthan 100 microseconds, depending on the particulars of the design.

FIG. 6D shows a central section view of a fourth embodiment of beamsteering mechanism 70. A silicon substrate 196 is processed viaphotolithography and MEMs fabrication techniques to have a cavity 198,cantilevered bending element 200, and an angled mirror surface 204. Avertical cavity surface emitting laser (VCSEL) 206 is mounted to thebottom surface of silicon substrate 196. The output beam of VCSEL laser206 is typically circular, and does not normally need to be conditioned,but may have a focussing and/or collimating lens positioned in front ofthe beam in some designs. The VCSEL laser 206 optical output beam isdirected by angled mirror surface 204 to fall on the mirror surface 202of bending element 200. Mirror surfaces 202, 204 may be gold, silver,aluminum, nickel, titanium, or other reflective metal, and may be astack of several metal films. Mirror surfaces 202, 204 may also be amulti layer dielectric reflector formed of silicon dioxide, siliconnitride, sapphire, calcium fluoride, or other suitable dielectric film.In operation, a differential voltage is applied between interdigitalcontacts 208 to cause piezoelectric strain on the reverse side ofbending element 200. If the piezoelectric strain is compressive, thebending element 200 will bend up (above the top surface of substrate196). If the piezoelectric strain is tensile, the bending element 200will deflect down towards the interior of cavity 198. Depending on thesign of the differential voltage imposed on interdigital contacts 208,the strain will be compressive or tensile. In this manner, the outputoptical beam 210 may be swept through an angle θ as shown in thediagram. The angle θ is typically in the range of 17 degrees, and may beswept out in less than a millisecond, and in some cases, less than 100microseconds, depending on the particulars of the design.

FIG. 6E shows an isometric view of the structure shown in FIG. 6B, andthe pixellated far field pattern 165 which will be swept by the pulsedlaser transmitter 68. In operation, the advantages of the beam steeringmechanism 70 allow for a much lower peak power requirement for pulsedlaser transmitter 68. Shown here is a 1×8 line array of edge emittinglasers 152 mounted on first substrate 150, a rod lens 154, and bendingelements 158 having mirrored surfaces 160. An 8×8 far field pattern 165is shown here for simplicity. In effect, the entire energy 164 of eachedge emitting laser 152 may be directed to a single pixel in the fieldof view of the ladar sensor (32, 34). Each pixel subtends a certainsolid angle, and the purpose of the pulsed laser transmitter 68 is toilluminate the pixel in the field of view, and the ladar sensor willcalculate the range to each pixel thus illuminated. In the case of asolid state laser such as neodymium YAG erbium glass, the illuminatingenergy will be delivered in one giant pulse, and the entire far field165 must be illuminated. A typical detector array is a square array of128×128 pixels, for a total of 16,384. In the case of a 1 mJ laser, thiswould mean about 61 uJ delivered to each pixel in the far field. Theadvantage of the steerable laser transmitter is shown by this example. A61 uJ semiconductor laser is well within the realm of possibility,whereas a 1 mJ semiconductor laser would be quite impractical given thecurrent state of the technology. A similar structure (not shown) isassembled on the reverse side on second substrate 151, which caneffectively double the width of the illuminated space 165.

FIG. 7A shows an isometric view of an optical gain block 76 which may bepositioned in the ladar sensor receive path to amplify weak opticalreturn signals which are reflected from distant or low reflectanceobjects in the field of view. In this case, the optical gain blockcomprises a rectangular fused fiber bundle 212, which is created from anumber of individual erbium doped fibers 216. A diffuser plate 214 isbonded to each of the four sides, which will be illuminated by a pumplaser diode array.

FIG. 7B shows an isometric view of the assembly of optical gain block76. A pump laser VCSEL array 218 comprised of a number of VCSEL lasers220 is being bonded to the sub-assembly of fused fiber bundle 212 anddiffuser plates 214. Incoming reflected light enters each erbium dopedfiber 216 of the fused fiber bundle 212 at an exposed endface. The fibercross section is shown here as rectangular or square, but other fibercross sections may be used which are hexagonal, circular, etc.

FIG. 7C shows an end view of the completed assembly of optical gainblock 76, showing a pump laser VCSEL array 218 bonded to the outersurface of each diffuser plate 214. Electrical contacts 222 for anodeand cathode of each individual VCSEL 220 of the VCSEL array 218 arebrought to the outer surface of VCSEL array 218 by vias etched throughthe substrate and metal plated. Incoming reflected light enters eacherbium doped fiber 216 of the fused fiber bundle 212 at the exposedendface. The fiber cross section is shown here as rectangular or square,but other fiber cross sections may be used which are hexagonal,circular, etc.

FIG. 8 shows a central section view of a second embodiment of theoptical gain block 76. Instead of the erbium doped fiber bundle 212being side pumped as in FIGS. 7A-C, this design end pumps the erbiumdoped fibers 216 of the fused fiber bundle 212. A receiver housing 224has a cylindrical flange 226 which is a quick connect optical mount, andmay have interior threads for mounting an external lens assembly 228.Cylindrical flange 226 may alternatively provide a bayonet style opticalmount, which may be engaged by complementary features on the body oflens assembly 228. Lens assembly 228 may have a plurality of lenselements 230, shown here as a convex lens. However, other lens types maybe used, including concave, aspherical, and diffractive arrays for lenselement 230. Each lens element 230 is mounted inside a recess 232 formedon an interior surface of lens assembly 228. A plurality of O-rings 234may be used to provide compliance between the rigid lens elements 230and the metal body of lens assembly 228. In operation, reflected light236 passes through lens elements 230 and dichroic optical flat 238, andfalls on fused fiber bundle 212. Reflected light 236 is typically at the1.54 micron wavelength of erbium glass lasers, or in the 1530-1650nanometer band of semiconductor lasers formed in indium phosphide.Dichroic optical flat 238 passes a broad band of wavelengths in therange of 1530-1650 microns, and reflects light in a narrower bandcentered at 980 nanometers. A rectangular array of 980 pump laser diodes240 is positioned in a recess in the sidewall of receiver housing 224 atan angle to the propagation of reflected light 236. In this design, eachelement 242 of laser diode array 240 is a VCSEL producing an opticalbeam 244 which falls on diffuser 246 in response to an electrical drivesignal supplied through electrical contacts 241. Diffuser 246 istypically a diffractive grating or holographic lens element, whichuniformly distributes the light from each optical beam 244 uniformlyacross the output face of diffuser 246, shown here as a number ofsmaller optical output beams 248, each produced from a single input beam244. In this manner, a highly uniform column of light is produced whichwill uniformly illuminate each erbium doped fiber 216 of fused fiberbundle 212. Output beams 248 fall on the underside of dichroic opticalflat 238 and are reflected to the input side of fused fiber bundle 212.In this way, erbium doped fibers 216 which are fused together in fusedfiber bundle 212 are optically pumped with 980 nm optical energy. At theother end of fused fiber bundle 212 is another dichroic optical flat 250which again has the properties of passing broad band of wavelengths inthe range of 1530-1650 microns, and reflecting light in a narrower bandcentered at 980 nanometers. When the attenuated pump light exits fusedfiber bundle 212, it encounters the reflecting surface of dichroicoptical flat 250, and is returned through the fused fiber bundle for asecond pass, thereby increasing the efficiency of the optical pumpingprocess. The dichroic optical flat 250 also passes the reflected lightsignal which has been amplified by the erbium doped fibers 216 which areacting as erbium doped fiber amplifiers, having been optically pumped toan excited state by pump laser diode array 240. The pump laser diodearray 240 can be started 1.2 milliseconds prior to the emission of anilluminating pulse in order to allow for the maximum level of excitedstates in the erbium doped fibers 216 of fused fiber bundle 212,producing the maximum optical gain. The fluorescence time of erbiumdoped glass is 1.2 milliseconds, so the maximum gain would requireapproximately this much advance in the timing of the application of theelectrical drive signal to pump laser diode array 240 through electricalcontacts 241. New types of erbium doped fibers, doped withnanoparticles, can have doping levels in excess of 10%, compared to a2-3% level for older products. This allows for a much higher gain perlength of fiber. Products such as the DrakaElite fiber from DrakaCommunications, and QX erbium doped phosphate glass fibers from Kigre,Inc. exhibit such properties and are well suited to the present design.Also, a method of using ytterbium doping to sensitize the erbium doping,and using 1480 nm pump light, produces 26 dB of gain in only 8.8 cm(3.5″) of fiber length at L-band (1575-1630 nm). This method wouldrequire changing the wavelength of the pulsed laser transmitter 68, andchanges to the dichroic optical flats 238 and 250. In the paperentitled, “High-Gain Short-Length Phosphate Glass Erbium-Ytterbium-DopedFiber Amplifiers”, authors Ayman M. Samara, et. al., of theOptoelectronics and Optical Communication Center, Department of Physics,UNC Charlotte, Charlotte, N.C. describe the structure and methodology touse the ytterbium sensitized erbium doped fibers. Further, in the paperentitled, “The Gain Performance of Ytterbium Doped Fiber Amplifier”, byParekhan M. Jaff, et. al., at the Department of Physics, University ofSulaymania, Kurdistan Region, Iraq, the performance of a ytterbium-onlydoped fiber is given which produces gains in excess of 20 dB for veryshort lengths of fiber, though again, the optimal pump wavelength shiftsto 910 nm, and the optimal pulse transmission wavelength shifts to 975nm. Again, changes are required to the wavelength of the pulsed lasertransmitter 68, and changes to the dichroic optical flats 238 and 250.Given appropriate levels of optical pumping, pump wavelength, pulsetransmission wavelength, and fiber composition, it is expected opticalgains in excess of 20 dB may be realized with a short (<6″) length oferbium doped fiber 216. For even higher gains, longer sections of fibermay be accommodated by the present design.

Finally, amplified reflected light 236 exits the output side of fusedfiber bundle 212 and falls on detector array 78. Detector array 78 isshown mounted to amplifier array 80, which is in turn mounted to acircuit substrate 252. Circuit substrate 252 is held in place by fourscrews 254, though other fasteners may be used, including staking withdeformable plastic, solder reflow, rivets, metal clips, and oradhesive/epoxy systems. On the reverse side of circuit substrate 252 ismounted readout integrated circuit 82 which connects through vias incircuit substrate 252 to amplifier array 80. Alternatively, the focalplane array (FPA) assembly consisting of detector array 78, amplifierarray 80, circuit substrate 252, and readout IC (ROIC) 82 may bereplaced by any one of a number of different FPA options detailed inFIGS. 11-17.

FIG. 9 shows a central section view of a third embodiment of the opticalgain block 76. The receiver housing 224, cylindrical flange 226, lensassembly 228, lens elements 230, dichroic optical flat 250, circuitsubstrate 252, fasteners 254, detector array 78, electronic amplifierarray 80, and ROIC 82 are of similar or identical design as in thedesign of FIG. 8, though the length of receiver housing 224 has beenreduced. Replacing fused fiber bundle 212 is vertical cavitysemiconductor amplifier (VCSOA) array 256. VCSOA array 256 is formedatop an indium phosphide substrate, and the structure and formation isdescribed in detail in association with FIG. 10. In an alternativedesign, the indium phosphide substrate may be removed via chemicalmechanical polishing (CMP) and a glass optical flat substituted. Theindividual VCSOA elements 264 optically amplify incoming reflected light236. The VCSOA 264 optical gain region is electrically pumped throughelectrical connections 258. Electrical connections 258 in a preferredembodiment are made by a ribbon cable with an insulator 260 around eachconductor of electrical connections 258. Connections to the reverse sideof VCSOA array 256 are made by through substrate vias 262. In operation,dichroic plate 250 acts as an optical filter, keeping ambient visibleand infrared light from reaching detector array 78. An optionalmechanical shutter 257 may be operated electrically by actuator 259which is connected to the system control processor 58 by wire(s) 263,which are insulated from the housing of lens assembly 228 by insulator261. Mechanical shutter 257 may be a venetian blind type, a rotary iris,a clapper, or a slide. Actuator 259 may be a linear actuator, or a smallelectric motor or stepper motor with rotary output, depending on thetype of shutter specified in the design.

FIG. 10A shows a central section view of a VCSOA element 264. Incomingreflected light 236 enters the VCSOA through an antireflection (AR)coating 266, and passes through substrate 268 which may be indiumphosphide, or in an alternative embodiment, an optical flat. DistributedBragg Reflector (DBR) 270 at the top of the cavity attenuates reflectedlight 236 before it enters gain region 272 which is a multiple quantum(MQW) well material tuned to the wavelength of the light pulsetransmission and reflection. The reflected light signal 236 is thenamplified in the gain region which has been electrically pumped throughanode 278 and cathode 280 electrical contacts. A DBR 274 at the bottomof the cavity confines the optical mode, allowing for multiple passesthrough the gain region, creating a type of resonant amplifier. Finally,an AR coating 276 provides for optimal transmission to the detectorarray 78. Some care must be exercised not to pump the cavity too hard,or else a lasing mode may be excited. The structure may be thought of asa VCSEL operated below the lasing threshold. The DBRs may be formed inindium phosphide (InP), which has been been doped to produce acontrasting index of refraction, or the structure may be formed inindium gallium arsenide (InGaAs), gallium arsenide (GaAs), galliumnitride (GaN), or other suitable opto-semiconductor, depending on thewavelength of transmission.

FIG. 10B shows the manner in which the VCSOA elements 264 may be formedby wafer bonding. In this process, an optical flat 268 is the startingpoint, and AR coating 266 is deposited on the top surface. On the bottomsurface of optical flat 268, top DBR 270 is formed by alternating layersof dielectric material, which may be any optical material, such asborosilicate glass, silicon dioxide, silicon nitride, titanium oxide,magnesium fluoride, zinc sulfide, indium antimonide, etc. These filmsare typically applied by physical vapor depoition (PVD). An InP waferhaving on the top surface a MQW gain region 272 is then bonded to thebottom side of optical flat 268, and the majority of the exposed InPsubstrate is then removed via CMP. A bottom DBR 274 is then formed byalternating layers of dielectric material in a similar manner to top DBR270. Finally, AR coating 276 is applied, mesas etched, and a metalpattern deposited to form anode 278 and cathode 280 electrical contacts.

FIG. 11 shows a central section view of a preferred embodiment of afocal plane array having a detector array 78. In this design, detectorarray 78 is formed on an inexpensive gallium arsenide (GaAs) substrate282. A thin metamorphic layer 284 of graded indium gallium aluminumarsenide (InGaAlAs) is grown on the GaAs substrate to create a crystalstructure lattice matched to InGaAs. A thin layer of InGaAs 286 is thengrown epitaxially. Finally, a stack of epitaxial layers are grown inInGaAs, and doped to be p-type, n-type, or intrinsic. This layer stackis then etched to form mesas 288. The epitaxial stackup may provide aPIN, NIP, or APD detector type. Shown here is a n-up PIN configuration,with the n-type material illuminated through the substrate, and thep-type material at the apex of each mesa 288. Shown in this figure are12 detectors for the purpose of clarity, though an array having 128×128detector elements 288 is typical. Metal is then deposited in a gridpattern, creating a common cathode contact 290. The anode connections ofeach detector element 288 are made through indium bumps 292 to anamplifier input of a unit cell of the readout integrated circuit (ROIC)82. Indium bumps 292 may be cold bonded, or may in an alternativeembodiment be made of solder and reflowed to make electricalconnections. Readout integrated circuit 82 has a number of throughsubstrate vias (TSVs) 294 which connect the circuitry on the top of ROIC82 to the reverse side. Bonding pads 296 may be used to make someelectrical connections through wirebonds 298 to conductive pads 300 onsupporting circuit substrate 302. Other electrical connections are madefrom ROIC 82 to circuit substrate 302 through solder bumps 304. The ROIC82 die may be eutectically attached to the circuit substrate 302 bysolder 306 or by a thermally conductive epoxy in the alternative.

FIG. 12 shows a central section view of a preferred packaging design ofa focal plane array having a detector array 78. Detector array 78 may bean M×N rectangular array where N and M may be anywhere from 2 to severalhundred pixels on each axis. Circuit substrate 302 is typically aceramic; either alumina, aluminum nitride, or beryllium oxide, dependingon the application. A conductor 314 is formed typically by an additivethick film process, where conductive inks are applied through a silkscreen mask or stencil and then fired at an elevated temperature.Conductor 314 may also be formed in a thin film process, where theentire top surface of circuit substrate 302 is covered by a conductivefilm which is deposited by sputtering or physical vapor deposition(PVD). The surface of circuit substrate 302 is then patterned withphotoresist and etched as in a typical printed circuit board (PCB)process. A thick film insulating layer 308 is then printed using silkscreen or stencil, and fired. Openings are left in insulating layer 308for filled conductive vias 310, which are then printed and fired in thesame sequence as picture frame shaped conductive pad 312. ROIC 82 withdetector array 78 attached is then eutectically attached to circuitsubstrate 302 with solder 306 or in the alternative, a thermallyconductive epoxy. Electrical connections are made from ROIC 82 tocircuit substrate 302 by solder bumps 304 which are reflowed prior tosealing of the assembly. A hermetic window cover comprised of flatwindow glass 320 retained in CoVar® frame 318 is next attached by acontinuous solder seam 316. Frame 318 is nickel and gold plated toenhance solderability. Window glass 320 may be other than flat, and mayhave a lens curvature. Window glass 320 may also have a filter appliedto a surface in the form of a thin optical film which rejects all lightbut the wavelength of interest; typically 1.54-1.57 microns in thepresent design. Frame 318 may be formed by extrusion or machining, ormay be formed in a deep draw process, where it may have a larger radiusin the corner where it meets with window glass 320, taking on the shapeof a rectangular tub. Window glass 320 is attached to frame 318 by afrit seal process, where a low temperature glass alloy is applied in afrit powder form and the assembly then heated until the glass fritpowder reflows, permanently attaching window glass 320 to frame 318. Inthis manner, a low cost, yet hermetic and high performance package forthe FPA assembly may be effected.

FIG. 13 shows a central section view of another preferred packagingdesign of a FPA having a detector array 78 which incorporates anamplifier array to substitute and augment the input amplifier associatedwith each unit cell of ROIC 82. The change from the design of FIG. 12 isa simple one, with the ROIC 82 being removed to the reverse side ofcircuit substrate 302, while the detector is now positioned atop atwo-dimensional amplifier array 80. ROIC 82 is typically fabricated in ahigh speed CMOS process. The process used has a very high density, and agreat deal of the available unit cell area is consumed by samplingcircuitry and analog memory cells. This limits the ability of circuitdesigners to realize a high gain transimpedance amplifier of the typewhich is embodied in amplifier array 80 in the instant design. Amplifierarray 80 performs two important functions; the first being theamplification of low level photocurrents received from detector elements288. Because the technology chosen for amplifier array 80 is a BiCMOSprocess, high forward gain amplifier circuits are possible. Having theentire unit cell area available for the transimpedance amplifier meanssufficient bias current is available to produce the desired gain,without violating power density rules for the amplifier array 80fabrication. The second important function performed by amplifier array80 is the transformation of the output impedance of the detectorelements 288 from a very high impedance to a much lower impedance,suitable for transmission through the substrate 302. The design of theamplifier array 80 pixel amplifier circuitry will be discussed inassociation with FIG. 17. Each pixel amplifier of the amplifier array 80has an input connected to a terminal of a detector element 288 ofdetector array 78 through an indium bump or solder bump 330, andtypically has a single-ended output connected via a TSV 322 to a solderbump 324 on the reverse side. Solder bump 324 is reflowed to connectwith a second TSV 326 in circuit substrate 302 which is connectedthrough another solder bump 328 to an input of a unit cell electricalcircuit of ROIC 82. Electrical connections to the amplifier array 80 maybe through metal pads 314 or TSVs 326, after the package is assembledand sealed as described in FIG. 12. Circuit substrate 302 is typically athick film ceramic; either alumina, aluminum nitride, or berylliumoxide, depending on the application, but may also be a thin filmcircuit. As in FIG. 12, a hermetic window cover comprised of flat windowglass 320 retained in CoVar® frame 318 is attached by a continuoussolder seam.

FIG. 14 shows a central section view of yet another preferred focalplane array design, incorporating a detector array 78 comprised of atwo-dimensional array of discrete detector elements 332. Each discretedetector element 332 is formed by an epitaxial growth of p-type, n-type,and intrinsic layers of InGaAs on an InP substrate using conventionalmethods, and mesas formed by etching in the “streets” between detectorelements 332. The discrete detector elements 332 are then tested andmarked, and singulated via scribe and break or by dicing. Individualdetector elements 332 may be as small as 0.2-0.4 mm in the currentdesign. The discrete detector elements 332 shown have both anode andcathode contacts on the top surface, which may be effected by throughsubstrate vias 352 or by lateral processing of the n-type and p-typelayers. The individual detector elements 332 may be graded forperformance, and are then placed atop amplifier array 80 by a pick andplace robot. Modern pick and place assembly robots have very highthroughputs, and are capable of handling components with features assmall as 0.2 mm. Each detector element 332 is connected to a pixelamplifier 336 of amplifier array 80 through solder bumps 334. Solderbumps 334 connect the anode and cathode of detector element 332 to apixel amplifier input and a bias voltage output distributed by the pixelamplifier to detector element 332. A through substrate via 338 connectsthe pixel amplifier outputs to the reverse side of amplifier array 80where it connects through a solder bump 340 to a metal pad 342 oncircuit substrate 302. A through substrate via 344 connects metal pads342 to the reverse side of substrate 302 and metal pads 346. Metal pads346 connect to a unit cell electrical circuit of readout IC 82 throughsolder bumps 348. In the gaps between discrete detector elements 332, acarbon loaded epoxy ink 354 is distributed and cured, providing opticalisolation between detector elements 332 of the detector array 78.Therefore, the potential for optical crosstalk between pixels in theimage is greatly reduced. Epoxy ink 354 may be spun on and photoimagedand developed in the same manner a photoresist might be applied, or itmay be applied using a syringe tip and robotic translation table. Epoxyink 354 may in an alternative deign be loaded with other IR absorbingmaterials such as germanium, etc. Metal pads 350 provide a site for theframe 318 of a hermetic window cover to be soldered in place as shown inFIGS. 12 and 13. Thus the reflected light pulse is detected andconverted to an electrical signal, amplified and conditioned fortransmission through a supporting circuit substrate, and connected tothe input of a readout IC which is designed to amplify, threshold, anddigitally sample the analog waveshape of the reflected light pulse. Allof this is accomplished in an optimal manner by the structure of FIG.14.

FIG. 15A shows a central section view of a preferred detector array 78design which is featured in the FPA packaging design of FIG. 13. Thisdetector array 78 is fabricated by a wafer bonding technique. A heavilydoped p-type InP substrate 364 has an InGaAs PIN structure formedepitaxially thereon, with n-type layer 358 grown over intrinsic layer360, which is formed over the p-type layer 362. This InP substrate 364is then bonded to silicon wafer 356 using hydrophilic bonding at hightemperature, or by a room temperature process using surface activation.The room temperature process is preferred, and is a feature of theMitsubishi wafer bonding equipment, MWB 04/06E. FIG. 15B shows the nextstep in the processing of the array, with the majority of the InPsubstrate removed by chemical mechanical polishing (CMP). Shown at thebottom of FIG. 15B is the remnant semiconductor film of p-type InP 364which has survived the CMP thinning process. This removal of materialallows for the formation of mesa structures 366 by patterning andetching of the bonded wafer using conventional semiconductor processtechniques as shown in FIG. 15C. Finally, ohmic contacts are formed, andmetal electrodes deposited for both the common cathode 368, and theindividual anode contacts 370 for each detector element 288 of thedetector array 78. The preferred metallization is Ti/Pt/Au, thoughTi/Ni/Au or other schemes may be used. The structure may be inverted, ormay be an avalanche photodiode for greater sensitivity. In operation, apositive voltage bias of 2-10 VDC is applied to the cathode of eachdetector element 288 through the cathode contacts 368 seen at theperiphery of the array and in the gaps between mesas which form a 2Dgrid pattern. The anode contact 370 is connected to the input of a pixelamplifier, and the PIN diode operated in reverse bias mode. This InGaAsPIN on silicon detector array 78 allows for a simplified and morereliable assembly process. Normally, InGaAs PIN diodes must be grown ona lattice matched InP substrate. However, the silicon ROIC 82 to whichthe detector array 78 is then bonded has a much different coefficient ofthermal expansion (CTE) than the InP. Because service temperatures canvary over a 100° C. range, lateral stresses can accumulate and breaksolder bonds in a conventional InP/silicon ROIC hybrid FPA. Indiumremains ductile even at very low temperatures of −55° C., which is whyindium bonding is a preferred option in a typical InP/silicon ROICcombination. The new design of FIG. 15 creates a hybrid InGaAs detectorarray 78 on silicon substrate 356 which has the CTE of silicon, allowingfor the use of conventional solder techniques to bond detector array 78to silicon ROIC 82 or amplifier array 80.

FIG. 16 shows a top view of the packaging of FIG. 13 with the windowcover removed. Circuit substrate 302 has a picture frame shapedmetallization pattern 312 suitable for soldering the frame 318 of thewindow cover in place. Detector array 78 is bonded to amplifier array80, and a number of capacitors 372 are shown inside the packageperimeter, which are useful for decoupling the bias voltages supplied todetector array 78 and amplifier array 80. Also shown are a number ofresistors 374 and inductors 376. The resistors 374 may be used to setthresholds, adjust output levels, etc. The inductors 376 can be used asfilter elements, or to create peaking in pulse circuits. Otherelectronic components such as crystals, diodes, etc, may be usefulinside the detector array 78 packaging, and may be placed and solderedin a similar manner.

FIG. 17 shows the details of a preferred design of a pixel amplifier 336of amplifier array 80. The detector element 288 is a preferred design isa PIN diode, and is modelled in the SPICE simulation by the componentswithin dashed line 288. The photocurrent I1 typically varies between2-100 uA, and the parasitic capacitance of the PIN diode under reversebias is typically 0.3 pF or less. A differential amplifier consisting ofNPN transistors Q3 and Q4 provide sufficient gain to produce thetransimpedance gain of 75 kΩ through resistor R2. Completing thedifferential amplifier structure is variable current source I2, speedupcapacitor C2, and the base circuit of Q4. The current through I2 isnominally set to 1.2 mA, but may vary widely to accommodate changes inthe desired gain. The base circuit of Q4 is just a variable voltagegenerator V_(OFF), in series with resistor R10. The collector of Q4 isconnected directly to the +5 VDC supply, The collector of Q3 isconnected to variable current source I3, and to the second stageamplifier input, which is just the emitter of Q8. In operation, thedetector element 288 has a reverse bias leakage current (dark current)which may vary somewhat from one detector element to another. Thisvariation in bias current produces an offset voltage which may be nulledout by adjustment of the variable voltage generator V_(OFF). V_(OFF) istypically set at 1.55 VDC, but may easily change +/−10% to compensatefor variations in detector elements 288. VOFF may also change tocompensate for the effects of temperature and age on the amplifiercircuit and on detector elements 288. Each of these sources of DC errormay be compensated for by the circuit of FIG. 17 by proper adjustment ofV_(OFF) by the V_(CTRL) output of gain and offset adjustment circuit378. Resistor R2 is typically 75 kΩ, but may be changed to accommodatedifferent applications, ranging from as low as 5 kΩ to as high as 125kΩ. An optional inverting buffer amplifier stage may be used to adaptthe pixel amplifier 336 to legacy ROIC 82 inputs. This optionalinverting stage is comprised of transistor Q8 connected in series withresistor R7 as a diode bias for inverting amplifier Q7. R7 may bereplaced with a small 5-10 uA current source in some designs. Theemitter of Q7 is connected to a 1.14 VDC bias voltage; in this caseformed by the resistive divider of R1 and R3. The collector of Q7 is theoutput, and is connected through a resistor R8 to the positive biassupply. In the present design, +5 VDC is available, and is used toprovide higher performance, but the design has been tested with 3.3 VDCand 2.5 VDC supply voltages, though gain, offsets and bias setpointsmust be adjusted to provide adequate headroom. In operation, the DCerrors are nulled by varying the V_(CTRL) input to the adjustmentcircuit 378 while the detector array 78 is under bias and darkened by amechanical shutter 257. The detector array 78 may also be partiallydarkened by removing the electrical drive to the VCSOA array 256 insystems which are so equipped. Once the offsets are nulled, the value islatched in a memory cell within adjustment circuit 378 by a transitionon the LATCH input.

The setpoints of the gain curve for each pixel amplifier 336 may then beestablished by applying a diffused laser pulse at the receive lens ofthe ladar sensor, or at the subassembly level. One useful method todetermine the gain setpoints is to flood the detector array 78 withuniform pulsed illumination at two, or three levels; typically amaximum, a low level, and a mid-range level. The gain of the pixelamplifier is set by the amount of current provided by voltage controlledcurrent source I2, and the current through voltage controlled currentsource I3 is set to track the changes in I2 to create optimum load andbias conditions at the collector of Q3. At each level, each pixelamplifier 336 is selected in turn by activating the Row Select (R_(SEL))and Column Select (C_(SEL)) outputs, and reading the output electricalsignal level through the ROIC 82. A gain adjustment is then calculatedby control processor 58, and the gain adjusted in the pixel amplifier bythe G_(CTRL) input. The G_(CTRL) input, as well as other inputs toadjustment circuit 378 may be connected directly to control processor58, or may be passed through ROIC 82. The MODE input is set tocalibration mode, and the G_(CTRL) value latched in adjustment circuit378 by a transition on the LATCH input. Other setpoints on the gaincurve for pixel amplifier 336 are determined in the same manner at theone or two remaining optical input power levels, depending on thecalibration mode selected. The MODE line is toggled to access theseother setpoints. In the fast gain mode, the MODE select line is set tosensitivity time control (STC) mode. When STC mode is selected, theinitial gain is set to the lowest value to reduce the possibility ofstrong reflections from the near field saturating a number of pixels,and bleeding over into adjacent pixels, sometimes called “blooming”, anddiscussed in association with FIG. 1. At the instant the illuminatinglaser pulse is transmitted, the G_(TRL) input voltage is driven higher,and the gain is rapidly ramped from a first low gain setpoint to asecond high gain setpoint. This ramp is done over a short ramp timeT_(R), which is determined by the range at which maximum amplifier gainis desired. T_(R) is often as short as 200 nS for short range systems,but may be as long as 1-10 uS for systems with greater range capability.Typically a range of 1000 feet would require a 2 uS two way time offlight, so the gain would be ramped to the maximum value in 2 uS orless, depending on the power transmitted in the illuminating laserpulse.

In the preferred embodiments described herein, a number of digitalprocessors have been identified, some associated with the host vehicle,some associated with the ladar subsystem, and some associated with theindividual ladar sensors. The partitioning and the naming of thesevarious digital processors has been made based on engineering judgment,but other partitioning and naming conventions may be used withoutchanging the scope or intent, or affecting the utility of the invention.The function of those processors associated with the vehicle; thevehicle CPU 48, and the collision processor and airbag control unit 44,may be combined in a single digital processor in some futureembodiments. A combined vehicle CPU 48 and collision processor andairbag control unit 44 may also incorporate ladar system controller 30,which is normally associated with the ladar subsystem. The ladar systemcontroller 30 (including control processor 58) may in some alternativeembodiments be eliminated as a standalone circuit, and those functionsnormally performed by ladar system controller 30, as described hereinwould then be assumed by a more powerful vehicle CPU 48. Likewise, theobject tracking processor 98 of the individual ladar sensor could beabsorbed into the vehicle CPU 48, as could other ladar sensor processorssuch as the data reduction processor 86 and control processor 58. Thiswould follow a trend toward greater centralization of the computingpower in the vehicle. A trend towards decentralization may also takeplace in reverse, some alternative embodiments having ever more of theprocessing power pushed down into the ladar sensor subsystem (FIG. 4).In other alternative embodiments, perhaps in a robotic vehicle whereonly a single ladar sensor might be installed, substantially all of theprocessing power could be incorporated in the individual ladar sensoritself. The term digital processor may be used generically to describeeither digital controllers or digital computers, as many controllers mayalso perform pure mathematical computations, or perform data reduction,and since many digital computers may also perform control operations.Whether a digital processor is termed a controller or a computer is adescriptive distinction, and not meant to limit the application orfunction of either device.

Having now described various embodiments of the disclosure in detail asrequired by the patent statutes, those skilled in the art will recognizemodifications and substitutions to the specific embodiments disclosedherein. Such modifications are within the scope and intent of thepresent disclosure as defined in the following claims.

The invention claimed is:
 1. A ladar system mountable to a vehicle, saidsystem comprising: a laser transmitter comprising at least onesemiconductor laser with a pulsed laser light output configured totransmit light through a transmitting optic adapted to illuminate areflecting surface in a field of view; a laser drive circuit connectedto said at least one semiconductor laser and adapted to electricallydrive said at least one semiconductor laser in a predetermined sequence;a time zero reference circuit having a time zero reference electricaloutput adapted to signal the beginning of a pulsed laser lighttransmission; receiving optics adapted to collect and condition thepulsed laser light reflected from the reflecting surface; an opticalgain block with an input face positioned to receive the collected andconditioned pulsed laser light, optically amplify the pulsed laserlight, and transmit the optically amplified pulsed laser light to anoutput face; a two-dimensional array of light sensitive detectorspositioned to intercept light from said output face of said optical gainblock, each of said light sensitive detectors positioned to intercept apixellated portion of the pulsed laser light reflected from thereflecting surface, and each light sensitive detector having an outputproducing an electrical response signal; a detector bias circuitconnected to a voltage distribution grid of said array of lightsensitive detectors; and a readout integrated circuit with a clockcircuit and a plurality of unit cell electrical circuits; each of saidunit cell electrical circuits having an input connected to said clockcircuit and to said time zero reference electrical output, and having anamplifier with an input connected to one of said light sensitivedetector outputs, and each amplifier having an output, and a pulsedetection circuit connected to said amplifier output, and said pulsedetection circuit having a termination output, and a counter connectedto the time zero reference electrical output and to said clock circuit,said counter started counting by the time zero reference electricaloutput, and said counter connected to, and stopped counting by thetermination output, and said counter having an output proportional tothe distance to the reflecting surface.
 2. The ladar sensor of claim 1wherein said optical gain block is selected from the set of anerbium-doped fiber amplifier and a semiconductor optical amplifier. 3.The ladar sensor of claim 1 wherein said laser transmitter is an arrayof semiconductor lasers.
 4. The ladar sensor of claim 1 wherein saidladar sensor is integrated into an assembly selected from the set of aheadlight, a turn signal, taillight, parking light, and brake light. 5.The ladar system of claim 1 wherein said predetermined sequence is aline-by-line illumination of the field of view.
 6. The ladar system ofclaim 1 wherein said predetermined sequence is a pixel-by-pixelillumination of the field of view.
 7. The ladar sensor of claim 1wherein said two-dimensional array of light sensitive detectors isformed in a material selected from the set of silicon, indium galliumarsenide, indium gallium arsenide phosphide, aluminum gallium arsenide,and indium gallium nitride.
 8. The ladar sensor of claim 1 wherein saidtwo-dimensional array of light sensitive detectors is adapted to providemultiplication of at least one photoelectron.
 9. The ladar sensor ofclaim 1 wherein said two-dimensional array of light sensitive detectorsis selected from the set of an avalanche photodiode array and an imagetube focal plane array.